1. Field of the Invention
The invention relates generally to materials characterization equipment. In particular, the invention relates to electron analyzers of material probed by x-rays or electrons.
2. Background Art
Several types of analysis equipment have found widespread use in the characterization of materials, particularly near the material surface, by measuring the spectrum of relatively low-energy electrons emitted from the probed material, that is, electron spectroscopy of secondary electrons produced by probing radiation and thereafter emitted from the sample. In particular, such equipment is capable of determining the composition and electronic bonding structure of the surface material.
One such type of equipment involves x-ray photoelectron spectroscopy (XPS) in which keV x-rays irradiate the sample to produce electrons, more specifically photoelectrons, of somewhat lower energy, which are ejected from the sample and spectrally analyzed. Another type of equipment involves auger electron spectroscopy (AES) in which probe electrons in the keV to 10 keV energy range irradiate the sample to produce secondary electrons, more specifically Auger electrons, in the 100 eV to few keV energy range, which are emitted and spectrally analyzed.
Both types of equipment require determining the energy and intensity (flux) of keV electrons. However, such low-energy electrons are subject to very strong scattering by any matter between the probed sample and the electron analyzer. Particularly for AES but also for XPS, even the probing radiation is subject to strong scattering and absorption. As a result, conventional analysis equipment of this type has enclosed the probe source, the sample, and the detector in a high-vacuum chamber, for example held at 10xe2x88x928 Torr or less (1 Torr equals 133 Pa), commonly referred to as ultra-high vacuum (UHV), although 10xe2x88x926 Torr will be considered as the maximum pressure for an electron analyzer in some configurations. In particular, it has been considered infeasible to use a vacuum window to pass the low-energy radiation, particularly the electrons, between the sample and the detector so that the sample must be inserted into the same UHV chamber required for the low-energy electron optics and detector. As a result, conventional XPS and AES equipment has been characterized as being very large, weighing on the order of tons, and not amenable to remote operation. Nonetheless, the need has arisen for the use of such equipment for planetary exploration, for example, to probe the chemistry of the Martian landscape. XPS and AES provide the needed analysis, but at the present time the instrumentation is too large and heavy for applications in space.
Furthermore, various needs exist for electron spectroscopy of samples held in a gaseous environment at moderate pressures rather than at the UHV pressure required with conventional XPS and AES systems. First, even disregarding the weight issue, simulation of Martian chemistry and testing of satellite equipment on earth would benefit from performing the test analysis in a simulated Martian environment, which is dominated by CO2 and N2 but with little O2 and very little water, a completely different environment than Earth""s and undoubtedly resulting in a vastly different chemistry. Secondly, analysis of biological samples at UHV is suspect because there is always a question whether previously living tissue or organisms radically alter when exposed to UHV. In particular, most organisms and tissue exist in an aqueous environment, but water evaporates at room temperature at pressures of 20 Torr and less. It would be greatly advantageous to perform the spectroscopy with samples exposed to a 20 Torr room-temperature ambient or even at 15xc2x0 C. and a 10 Torr pressure. Thirdly, there is great interest to investigate gas-phase catalysis to determine the chemistry of reactions between a gas and a solid catalyst. Clearly, UHV pressures are not consistent with reasonable concentrations of the gas phase to be measured. Fourthly, it would be beneficial to directly study the chemistry of chemical vapor deposition (CVD) commonly used in the semiconductor industry, in which precursor gases react with and deposit reaction products on a substrate such as silicon wafer, thereby growing on the substrate a thin film of a material derived from the precursor. Many types of CVD are performed at moderate pressures of a few hundred milliTorr to tens of Torr. Accordingly, surface analysis performed at these pressures could directly measure the CVD process.
Recently x-ray sources have been developed which are vastly smaller and lighter than conventional x-ray guns. They are commercially available from Moxtek of Orem, Utah, Amptek of Bedford, Massachusetts, and Oxford Instruments. These compact sources have diameters of a few millimeters and include a thin transmissive or an obliquely aligned reflective metal target irradiated by electrons with energy of tens of keV to generate the desired x-rays. However, such small sources do not address the rest of the problem of heavy vacuum interlocks.
An electron window has recently been proposed for such high pressure electron spectroscopy. The window includes a number of thin walls with small apertures through them, which together with electron focusing permits electrons to travel from a higher pressure environment containing the sample to the UHV electron analyzer. Such equipment, however, has been very heavy and restricted to research environments.
An thin layer of window of thickness between 1 and 5 nm, more preferably between 2 and 3 nm, allows electrons having energy near a keV to pass therethrough with acceptable attenuation. The window layer is supported on a grid of much thicker ribs with the window layer extending in the apertures between the ribs to thereby provide mechanical strength to stand off the pressure difference.
An electron transmissive window may be formed by semiconductor processing techniques in which a window layer and a support layer are deposited on a substrate such as a silicon wafer or wafer chip. The support layer is photolithographically etched to form the ribs in the window area. The silicon wafer is photolithgraphically etched on it backside to form a window aperture surrounding the window area. The silicon wafer outside the window aperture may be used as a support. Deposition techniques include chemical vapor deposition, atomic layer deposition, sputtering, and for some materials oxidation, such as thermal oxidation.
Materials for the support and window layers include silicon oxide and silicon nitride, but other materials are possible.
In one embodiment, the window layer is deposited or otherwise formed over the substrate, and the support layer is deposited over the window layer. The support layer is etched selectively to the underlying window layer to form the grid. The substrate is backside etched selectively to the window layer.
In another embodiment, the support layer is formed over the substrate and photolithographically defined into the grid. The window layer is conformally or nearly conformally deposited over the ribs of the grid and the portions of the substrate exposed between the ribs. The substrate is backside etched selectively to the window layer. Preferably, a curved skirt portion of the window layer is formed in the corners of the ribs next to the then existing substrate.
A gas cell including an electron vacuum window may be fully inserted into a high-vacuum electron analyzer with an enclosed test sample but with a selected internal gas environment at a finite pressure significantly higher than the high vacuum. A sample stub with an electron vacuum window may project into the high-vacuum electron analyzer and allow a sample to be inserted to the area of the window from outside the analyzer. An electron vacuum window may be disposed on an exterior surface of the high-vacuum analyzer to allow manual placement of the sample next to the window. A simple vacuum enclosure may be placed over the sample and pumped to a medium vacuum or flooded with a controlled gas environment.